Because a steel rail (hereinafter referred to as a "rail") head suffers from contact friction with wheels of the vehicle and should bear a heavy load, it is the common practice to apply a heat treatment to the rail head so as to impart an excellent wear resistance thereto.
In order to impart an excellent wear resistance to a rail head through the heat treatment, it is known that the structure of the surface portion of the rail head should preferably be transformed into a uniform and fine pearlite structure. It is therefore necessary to transform the structure of the surface portion of the rail head, which is in contact with wheels of the vehicle, into a uniform and fine pearlite structure excellent in wear resistance to a prescribed depth inwardly from that surface. For the purpose of transforming the structure of the surface portion of a rail head into a fine pearlite structure to a prescribed depth inwardly from that surface, there are available a method known as the isothermal transformation feat treatment, which comprises keeping the rail head at the pearlite transformation temperature by mainly controlling a cooling arrest temperature, and another method known as the continuous cooling transformation heat treatment, which comprises cooling the rail head by mainly controlling a cooling rate. A typical temperature curve in the isothermal transformation heat treatment is shown by (A) in FIG. 1, and a typical temperature curve in the continuous cooling transformation heat treatment is shown by (B) in FIG. 1.
The rail head is cooled with the use of a cooling medium such as air, water, air-water mixture, boiling water, steam, or molten salt. These cooling media have respective problems as follows.